Grease composition and rolling bearing

ABSTRACT

A grease composition includes base oil, a thickener, and an organic modified polysiloxane. The base oil is an alkyl diphenyl ether. The thickener is a urea thickener, and the grease composition contains 10 to 30 mass % of the thickener based on the total amount of the base oil and the thickener. The grease composition contains 0.2 to 10 mass % of the organic modified polysiloxane based on the total amount of the base oil and the thickener.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2018-190187 filed on Oct. 5, 2018 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to grease compositions and rolling bearings having the grease composition sealed therein.

2. Description of Related Art

Bearings used in automotive electrical components such as alternators, electromagnetic clutches for car air conditioners, intermediate pulleys, and electric fan motors, and automotive engine auxiliary devices, etc. are used in harsh environments such as high temperature, high speed, high load, and high vibration. In rolling bearings that are used in such harsh environments, early flaking associated with a microstructural change of steel sometimes occurs in a locking collar and rolling elements during use of the bearing.

Grease intended to solve the early flaking problem has been reported. For example, International Patent Publication No. WO 94/03565 and Japanese Unexamined Patent Application Publication Nos. 2004-108403 and 2012-233158 disclose greases containing a urea thickener and an extreme pressure additive.

SUMMARY

In recent years, rolling bearings have been used under harsher conditions, and early flaking are more likely to occur in the rolling bearings. Such early flaking associated with a microstructural change of steel is completely different from subsurface-initiated flaking and is characterized by white microstructures. Such early flaking is called white structure flaking. The mechanism of causing this white structure flaking is mainly an increase in internal stress due to slipping, high surface pressure, impact load, etc. Moreover, a tribochemical reaction etc. occurs on the nascent surfaces exposed by friction between inner and outer rings and rolling elements, producing hydrogen by decomposition of water and grease. This hydrogen penetrates the bearing steel, further facilitating the white structure flaking.

It has been proposed to restrain occurrence of such white structure flaking by reducing friction and impact load. In the proposed method, friction is reduced by adding a specific extreme pressure additive, and impact load is reduced by increasing the thickness of a grease film on the surfaces of rolling elements, as described above. However, this method cannot reliably avoid the white structure flaking. Grease that is more suitable for avoiding such early flaking has therefore been desired.

The present disclosure provides a grease composition capable of restraining occurrence of white structure flaking in a rolling bearing, and a rolling bearing.

A grease composition according to a first aspect of the present disclosure includes: base oil, a thickener, and an organic modified polysiloxane. The base oil is an alkyl diphenyl ether. The thickener is a urea thickener, and the grease composition contains 10 to 30 mass % of the thickener based on a total amount of the base oil and the thickener. The grease composition contains 0.2 to 10 mass % of the organic modified polysiloxane based on the total amount of the base oil and the thickener.

When used in a rolling bearing, the grease composition according to the first aspect of the present disclosure can restrain occurrence of white structure flaking in the rolling bearing. The reason for this is as follows. Grease comprised of the grease composition contains a predetermined amount of organic modified polysiloxane. The grease therefore has excellent wettability with steel. Accordingly, when the grease is used by being sealed in a rolling bearing, the grease can easily enter between each rolling element and rolling surfaces of inner and outer rings. The grease can therefore easily form a grease film on rolling contact portions between each rolling element and the inner and outer rings and can continuously contribute to lubrication of the rolling contact portions without causing oil film shortage. The rolling bearing having the grease sealed therein can therefore reduce an increase in internal stress in the rolling contact portions. Moreover, since the grease continuously contributes to lubrication, the grease can restrain formation of nascent surfaces due to wear and can thus restrain production of hydrogen by the tribochemical reaction that occurs on the exposed nascent surfaces. The grease can thus prevent an increase in internal stress in the rolling contact portions, restrain production of hydrogen, and restrain occurrence of white structure flaking.

In the grease composition according to the first aspect of the present disclosure, the organic modified polysiloxane may be at least one selected from aralkyl modified polymethyl alkyl siloxanes and polyether modified polydimethylsiloxanes.

The grease composition according to the first aspect of the present disclosure can restrain occurrence of white structure flaking in a rolling bearing when used in rolling bearings that are used in harsh environments.

A rolling bearing according to a second aspect of the present disclosure includes a grease composition sealed in the rolling bearing. The grease composition contains base oil, a thickener, and an organic modified polysiloxane. The base oil is an alkyl diphenyl ether. The thickener is a urea thickener, and the grease composition contains 10 to 30 mass % of the thickener based on a total amount of the base oil and the thickener. The grease composition contains 0.2 to 10 mass % of the organic modified polysiloxane based on the total amount of the base oil and the thickener.

In the rolling bearing according to the second aspect of the present disclosure, the organic modified polysiloxane may be at least one selected from aralkyl modified polymethyl alkyl siloxanes and polyether modified polydimethylsiloxanes.

The rolling bearing according to the second aspect of the present disclosure can restrain occurrence of white structure flaking.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a sectional view of a ball bearing according to an embodiment of the present disclosure; and

FIG. 2 is a graph showing the evaluation results of cumulative hydrogen production of grease compositions prepared in examples and a comparative example.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present disclosure will be described with reference to the accompanying drawings. A rolling bearing according to the embodiment is a ball bearing with a grease composition according to the embodiment of the present disclosure sealed therein. FIG. 1 is a sectional view of a ball bearing according to an embodiment of the present disclosure. A ball bearing 1 includes an inner ring 2, an outer ring 3, balls 4 as a plurality of rolling elements, and an annular cage 5. The outer ring 3 is disposed radially outside the inner ring 2. The balls 4 are arranged between the inner ring 2 and the outer ring 3. The cage 5 retain the balls 4 therein. Seals 6 are provided on one side and the other side in the axial direction of the ball bearing 1. Grease G comprised of the grease composition according to the embodiment of the present disclosure is sealed in an annular region 7 between the inner ring 2 and the outer ring 3.

The inner ring 2 has along its outer periphery an inner raceway surface 21 on which the balls 4 roll, and the outer ring 3 has along its inner periphery an outer raceway surface 31 on which the balls 4 roll. The plurality of balls 4 are interposed between the inner raceway surface 21 and the outer raceway surface 31 and roll on the inner raceway surface 21 and the outer raceway surface 31. The grease G sealed in the region 7 is also present in contact portions between each ball 4 and the inner raceway surface 21 of the inner ring 2 and contact portions between each ball 4 and the outer raceway surface 31 of the outer ring 3. The grease G is sealed in the ball bearing 1 so as to occupy 20 to 40% of the volume of the space surrounded by the inner ring 2, the outer ring 3, and the seals 6 excluding the volume of the space occupied by the balls 4 and the cage 5. The seals 6 are annular members including an annular cored bar 6 a and an elastic member 6 b fixed to the cored bar 6 a. Each seal 6 has its radially outer portion fixed to the outer ring 3 and its radially inner portion slidably attached to the inner ring 2. The seals 6 suppress leakage of the sealed grease G to the outside.

The ball bearing 1 configured as described above has sealed therein the grease G comprised of the grease composition according to the embodiment of the present disclosure. The rotation torque of the ball bearing 1 is therefore small and the ball bearing 1 has a sufficient lubrication service life. Moreover, such white structure flaking as described above is less likely to occur in the ball bearing 1.

Next, the grease composition that forms the grease G will be described in detail. The grease composition that forms the grease G is the grease composition according to the embodiment of the present disclosure and contains base oil, a thickener, and an organic modified polysiloxane. One of technical features of the grease composition is that the grease composition contains an organic modified polysiloxane. Since the grease composition contains an organic modified polysiloxane, white structure flaking can be restrained from occurring in the rolling bearing. This is because the grease composition containing an organic modified polysiloxane can prevent an increase in internal stress at the rolling contact portions and restrain formation of nascent surfaces due to wear, as described above.

In the grease composition, the base oil is an alkyl diphenyl ether. The grease composition is therefore suitable for providing grease with satisfactory heat resistance and flaking resistance. This alkyl diphenyl ether can be a known alkyl diphenyl ether that is used as base oil of grease for rolling bearings.

For example, the alkyl diphenyl ether may be an alkyl diphenyl ether having the following structural formulas (1) to (3).

(In the formula (1), R represents an alkyl group. R is bonded to a benzene ring.)

(In the formula (2), R_(a) and R_(b) represent an alkyl group. Each of R_(a) and R_(b) is bonded to a benzene ring. R_(a) and R_(b) may be the same or different.)

(In the formula (3), R_(n) represents an alkyl group, and m is an integer of 3 to 10. Each R._(n) is bonded to a benzene ring. Each of the plurality of R_(n)s may be the same as or different from the remainder of R_(n)s.)

In the grease composition, the thickener is a urea thickener. Examples of the urea thickener include urea compounds such as diurea, triurea, tetraurea, and polyureas (except diurea, triurea, and tetraurea), urea-urethane compounds, urethan compounds such as diurethan, and mixtures thereof. The urea thickener is preferably diurea represented by the following structural formula (4).

R₁—NHCONH—R₂—NHCONH—R₃   (4)

(In the formula (4), R₁ and R₃ are independent of each other and represent an amino residue and R₂ represents a diisocyanate residue). Diurea represented by the above structural formula (4) is a reaction product of an amine compound and a diisocyanate compound.

Examples of the amine compound include alkylamines, alkyl phenyl amines, and cyclohexylamines. Examples of the diisocyanate compound include aliphatic diisocyanates, alicyclic diisocyanates, and aromatic diisocyanates.

A specific example of diurea is a reaction product of a mixed amine and a diisocyanate compound. The mixed amine is a mixture of an alkyl phenyl amine having an alkyl group with 8 to 16 carbons and a cyclohexylamine. In the alkyl phenyl amine, the alkyl group may be a straight-chain alkyl group or a branched alkyl group, and a phenyl group may be substituted by the alkyl group in any of ortho, meta, and para positions. Specific examples of the alkyl phenyl amine include octylanilines, decylanilines, dodecylanilines, hexadecylanilines, and isododecylanilines. The alkyl phenyl amine is preferably p-dodecylaniline because of its satisfactory dispersibility. The content of the cyclohexylamine in the mixed amine is preferably 91 to 99 mol % based on the total amount of the alkyl phenyl amine and the cyclohexylamine. This is suitable for producing a grease composition with satisfactorily formability of a lubricating film.

The diisocyanate compound is preferably an aromatic diisocyanate because it is suitable for producing a grease composition with satisfactory heat resistance. Examples of the aromatic diisocyanate include diphenylmetane-4,4′-diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, and 3,3′-dimethyldiphenyl-4,4′-diisocyanate. Among these, diphenylmetane-4,4′-diisocyanate and 2,6-tolylene diisocyanate are preferable in view of their availability, and diphenylmetane-4,4′-diisocyanate is more preferable due to its satisfactory heat resistance.

The reaction between the mixed amine and the diisocyanate compound can be caused under various conditions in order to obtain diurea represented by the structural formula (4). However, it is preferable to cause the reaction between the mixed amine and the diisocyanate compound in base oil as it produces a diurea compound with high uniform dispersibility as a thickener. The reaction between the mixed amine and the diisocyanate compound may be caused in either base oil produced by adding base oil having a diisocyanate compound dissolved therein to base oil having an alkyl phenyl amine and a cyclohexylamine dissolved therein or base oil produced by adding base oil having an alkyl phenyl amine and a cyclohexylamine dissolved therein to base oil having a diisocyanate compound dissolved therein.

The temperature and time for the reaction between the mixed amine and the diisocyanate compound are not particularly limited, and conditions similar to those that are typically used in this type of reaction can be used. The reaction temperature is preferably 60 to 170° C. in view of solubilities and volatilities of the mixed amine and the diisocyanate compound. The reaction time is preferably 0.5 to 2.0 hours in order to complete the reaction between the mixed amine and the diisocyanate compound and to reduce the time required to produce the grease composition. The reaction between the amino group of the mixed amine and the isocyanate group of the diisocyanate compound proceeds quantitatively, and it is preferable that the mixed amine and the diisocyanate compound react in a mole ratio of 2:1.

Diurea represented by the above structural formula (4), which is a reaction product of the mixed amine and the diisocyanate compound, is a mixture of diurea in which both R₁ and R₃ in the formula (4) are alkyl phenyl amine residues, diurea in which both R₁ and R₃ are cyclohexylamine residues, and diurea in which one of R₁ and R₃ is an alkyl phenyl amine residue and the other is a cyclohexylamine residue.

The content of the thickener in the grease composition is 10 to 30 mass % based on the total amount of the base oil and the thickener. When the content of the thickener is less than 10 mass %, the thickener cannot sufficiently hold the base oil. Accordingly, when this grease composition is used by being sealed in a rolling bearing, the grease composition may be scattered or leak from the rolling bearing. When the content of the thickener is higher than 30 mass %, the grease composition is hard. Accordingly, when this grease composition is used by being sealed in a rolling bearing, the torque of the rolling bearing may be increased, and the seizure life of the rolling bearing may be reduced due to reduced fluidity of the grease composition. The content of the thickener is preferably 11 to 18 mass % based on the total amount of the base oil and the thickener.

The grease composition further contains an organic modified polysiloxane. The grease composition therefore has excellent wettability with steel. Accordingly, when this grease composition is used by being sealed in a rolling bearing, the grease composition can easily enter between the friction surfaces such as between the inner ring and the rolling elements and between the outer ring and the rolling elements. Occurrence of white structure flaking can thus be restrained.

An example of the organic modified polysiloxane is a polysiloxane having a side chain modified with an organic group as represented by the following structural formula (5).

(In the formula (5), R⁴ represents an alkyl group, m represents a positive integer, and n represents 0 or a positive integer). In the structural formula (5), R⁴s may be the same or different, and the organic groups may be the same or different.

Examples of the organic group that modifies the side chain of the polysiloxane represented by the structural formula (5) include an aralkyl group and a (poly)ether group having one or more ether bonds. The aralkyl group is not particularly limited as long as an aryl group such as a phenyl group is substituted for one of hydrogen atoms of an alkyl group. Specific examples of the aralkyl group are aralkyl groups represented by structural formulas such as —(CH₂)_(a)—Ph (where Ph represents a phenyl group and a represents an integer of 1 or greater) and —(CH₂)_(b)—CH(CH₃)—Ph (where Ph represents a phenyl group and b represents 0 or an integer of 1 or greater).

Examples of the (poly)ether group include functional groups having an alkylene oxide group such as an ethylene oxide group “—(C₂H₄O)—” or a propylene oxide group “—(C₃H₆O)—.” A specific example of such functional groups is a functional group represented by a structural formula such as —R₅(C₂H₄O)_(c)(C₃H₆O)_(d)R₆ (where R₅ represents a single bond, an alkylene group, or an alkylene oxide group, R₆ represents hydrogen, an alkyl group, or an aryl group, c and d are independent of each other and represent 0 or an integer of 1 or greater (except the case where both c and d are 0).

The organic modified polysiloxane is preferably at least one selected from aralkyl modified polymethyl alkyl siloxanes and polyether modified polydimethylsiloxanes as they are highly compatible with the base oil and are suitable for providing heat resistance required for grease compositions. A commercially available organic modified polysiloxane may be used.

Specific examples of the aralkyl modified polymethyl alkyl siloxanes include BYK-322 (made by BYK Additives & Instruments), BYK-323 (made by BYK Additives & Instruments), KF-410 (made by Shin-Etsu Chemical Co., Ltd.), X-22-2516 (made by Shin-Etsu Chemical Co., Ltd.), and X-22-1877 (made by Shin-Etsu Chemical Co., Ltd.).

Specific examples of the polyether modified polydimethylsiloxanes include BYK-330 (made by BYK Additives & Instruments), BYK-331 (made by BYK Additives & Instruments), KF-351A (made by Shin-Etsu Chemical Co., Ltd.), KF-352A (made by Shin-Etsu Chemical Co., Ltd.), KF-353 (made by Shin-Etsu Chemical Co., Ltd.), KF-354L (made by Shin-Etsu Chemical Co., Ltd.), KF-355A (made by Shin-Etsu Chemical Co., Ltd.), KF-615A (made by Shin-Etsu Chemical Co., Ltd.), KF-945 (made by Shin-Etsu Chemical Co., Ltd.), KF-640 (made by Shin-Etsu Chemical Co., Ltd.), KF-642 (made by Shin-Etsu Chemical Co., Ltd.), KF-643 (made by Shin-Etsu Chemical Co., Ltd.), KF-644 (made by Shin-Etsu Chemical Co., Ltd.), KF-6020 (made by Shin-Etsu Chemical Co., Ltd.), KF-6204 (made by Shin-Etsu Chemical Co., Ltd.), and X-22-2516 (made by Shin-Etsu Chemical Co., Ltd.).

The content of the organic modified polysiloxane in the grease composition is 0.2 to 10 mass % based on the total amount of the base oil and the thickener. When the content of the organic modified polysiloxane is less than 0.2 mass %, there is almost no effect of adding the organic modified polysiloxane. When the content of the organic modified polysiloxane is higher than 10 mass %, the grease composition is soft. Accordingly, when this grease composition is used by being sealed in a rolling bearing, the rolling bearing may have reduced resistance to leakage of the grease composition and the torque may be increased due to an increase in fluid resistance. The content of the organic modified polysiloxane is preferably 0.5 to 5 mass % based on the total amount of the base oil and the thickener.

Various additives such as an antioxidant, an extreme pressure additive, an anti-wear agent, a dye, a hue stabilizer, a thickening agent, a structural stabilizer, a metal deactivator, a viscosity index improver, and an antirust additive may be added as appropriate to the grease composition within such a range that does not reduce or eliminate the effect of the present disclosure. In the case where the grease composition contains such additives, it is preferable that the total content of the additives in the grease composition is 10 mass % or less based on the total amount of the base oil and the thickener.

The grease composition of the present disclosure can be used in those portions which require grease lubrication and is suitably used as grease for rolling bearings. The grease composition of the present disclosure is particularly suitable as grease for rolling bearings that are required to have resistance to white structure flaking. The grease composition is therefore suitably used as grease to be sealed in rolling bearings that are used in harsh environments, like automotive electrical components such as alternators, electromagnetic clutches for car air conditioners, intermediate pulleys, and electric fan motors, and automotive engine auxiliary devices, etc.

Next, a method for producing the grease composition will be described. For example, the grease composition can be produced by first preparing base grease comprised of base oil and a thickener, then adding the organic modified polysiloxane and any desired additive to be contained in the grease composition as necessary to the base grease, and mixing them together by stirring in a planetary centrifugal mixer etc.

The applicable embodiment is not limited to the above embodiment and may be carried out in other forms. The rolling bearing according to the embodiment of the present disclosure is not limited to the ball bearing having the grease composition according to the embodiment of the present disclosure sealed therein. The rolling bearing according to the embodiment of the present disclosure may any other rolling bearing using rolling elements other than balls, such as a needle bearing or a roller bearing, as long as the rolling bearing has the grease composition according to the embodiment of the present disclosure sealed therein.

The present disclosure will be described in more detail based on examples. However, the applicable embodiment is not limited to such examples. A plurality of grease compositions were prepared and characteristics of each grease composition were evaluated. The composition of each grease composition and the evaluation results are shown in Table 1.

The following materials were used in the examples and a comparative example.

Diisocyanate Compound

-   MDI: diphenylmetane-4,4′-diisocyanate

Amine Compounds

-   CHA: cyclohexylamine -   PDA: p-dodecylaniline

Base Oil

-   ADE: alkyl diphenyl ether (MORESCO-HILUBE LB-100 made by MORESCO     Corporation), dynamic viscosity at 40° C. is 102 mm²/s.

Organic Modified Polysiloxanes Aralkyl modified polymethyl alkyl siloxane (1): BYK-322 (made by BYK

Additives & Instruments)

Aralkyl modified polymethyl alkyl siloxane (2): BYK-323 (made by BYK Additives & Instruments)

Polyether modified polydimethylsiloxane (1): BYK-330 (made by BYK Additives & Instruments)

Polyether modified polydimethylsiloxane (2): BYK-331 (made by BYK Additives & Instruments)

COMPARATIVE EXAMPLE 1

(1) Amine compounds (CHA and PDA) as thickener materials were mixed with half of ADE corresponding to the base oil content shown in Table 1 so that the resultant mixture had the CHA and PDA contents shown in Table 1. The mixture was heated to 100° C. to dissolve CHA and PDA in ADE. A solution A was thus prepared.

(2) Aside from the solution A, a diisocyanate compound (MDI) as a thickener material was mixed with half of ADE corresponding to the base oil content shown in Table 1 so that the resultant mixture had the MDI content shown in Table 1. The mixture was heated to 100° C. to dissolve MDI in ADE. A solution B was thus prepared.

(3) The solution A was slowly added to the solution B while stirring the solution B. Thereafter, the resultant solution was kept at 150° C. for 30 min. The solution was then allowed to cool to a room temperature while continuing to stir.

(4) Lastly, the solution thus cooled to the room temperature was homogenized on a triple roll mill to produce a grease composition.

EXAMPLE 1

A mixture of base oil and a thickener was prepared by a process similar to that of (1) to (3) of Comparative Example 1, and BYK-322 was added to the mixture so that the resultant mixture had the BYK-322 content shown in Table 1. Thereafter, the mixture was mixed in a mixer at 2,000 rpm and then homogenized on a triple roll mill to produce a grease composition.

EXAMPLE 2

A grease composition was prepared by a process similar to that of Example 1 except that the amount of BYK-322 was changed so that the mixture had the BYK-322 content shown in Table 1.

EXAMPLE 3

A grease composition was prepared by a process similar to that of Example 2 except that BYK-323 was used as an additive instead of BYK-322.

EXAMPLE 4

A grease composition was prepared by a process similar to that of Example 2 except that BYK-330 was used as an additive instead of BYK-322.

EXAMPLE 5

A grease composition was prepared by a process similar to that of Example 2 except that BYK-331 was used as an additive instead of BYK-322.

(Evaluation of Grease Compositions)

The grease compositions prepared in Examples 1 to 5 and Comparative Example 1 were evaluated. The results are shown in Table 1 and FIG. 2.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 1 Base Oil Content (mass %)*¹ 85.4 85.4 85.4 85.4 85.4 85.4 (ADE) Dynamic Viscosity at 40° C. (mm²/s) 102 Thickener Thickener Materials MDI 50 50 50 50 50 50 (Diurea) (Mole Ratio) CHA 95 95 95 95 95 95 PDA 5 5 5 5 5 5 Content (mass %)*² 14.6 14.6 14.6 14.6 14.6 14.6 Additive Type and Aralkyl Modified Polymethyl Alkyl Siloxane (1) 1 5 — — — — Content (mass %)*³ Aralkyl Modified Polymethyl Alkyl Siloxane (2) — — 5 — — — Polyether Modified Polydimethylsiloxane (1) — — — 5 — — Polyether Modified Polydimethylsiloxane (2) — — — — 5 — Worked Penetration (60 W) 384 391 410 379 334 388 Ratio of Cumulative Hydrogen Production*⁴ 0.31 0.29 0.61 0.66 0.72 1.00 *¹Content of base oil based on total amount of base oil and thickener *²Content of thickener based on total amount of base oil and thickener *³Content of additive based on total amount of based oil and thickener *⁴Ratio relative to hydrogen production in Comparative Example 1

Evaluation shown in Table 1 was carried out by the following method.

(1) Worked penetration (60W) was measured by a method in accordance with JIS K 2220-7.

(2) Ratio of Cumulative Hydrogen Production

A friction test of the grease compositions prepared in Examples 1 to 5 and Comparative Example 1 was conducted using an oscillating friction and wear tester (SRV-IV) made by Optimol Instruments, and the amount of hydrogen produced was measured as cumulative hydrogen production. Test conditions using the oscillating friction and wear tester are shown in Table 2.

TABLE 2 Test items Conditions Test Piece Upper Part Steel Ball (made of SUJ2, ϕ10 mm) Lower Part Circular Plate (made of SUJ2, ϕ24 × 7.8 mm) Load 160N (maximum Hertz pressure = 2.5 GPa) Temperature 80° C. Humidity 45% RH Amplitude 1,000 μm Frequency 50 Hz Test Time 30 min.

The cumulative hydrogen production was measured using Transpector CPM made by INFICON Co., Ltd, which is a quadrupole mass spectrometer. A special nozzle (1.5 meter capillary assembly kit) was attached to the quadrupole mass spectrometer, and the tip of the nozzle was placed near the test piece (steel ball and circular plate) on the oscillating friction and wear tester. Hydrogen produced during the test was thus measured. The measurement was performed under the following conditions.

Ionization Voltage: 70 eV

Ionization Current: 500 μA

Analysis Time: 30 min

In Table 2 and FIG. 2, hydrogen productions (cumulative hydrogen productions) of Examples 1 to 5 are shown as ratios relative to hydrogen production (cumulative hydrogen production) of Comparative Example 1.

As can be seen from the results of Examples 1 to 5 and Comparative Example 1, the use of the grease composition according to the embodiment of the present disclosure can reduce the cumulative hydrogen production. Accordingly, the grease composition sealed in a rolling bearing can restrain occurrence of white structure flaking of the rolling bearing. 

What is claimed is:
 1. A grease composition, comprising: base oil that is an alkyl diphenyl ether; a thickener that is a urea thickener, the grease composition containing 10 to 30 mass % of the thickener based on a total amount of the base oil and the thickener; and an organic modified polysiloxane, the grease composition containing 0.2 to 10 mass % of the organic modified polysiloxane based on the total amount of the base oil and the thickener.
 2. The grease composition according to claim 1, wherein the organic modified polysiloxane is at least one selected from aralkyl modified polymethyl alkyl siloxanes and polyether modified polydimethylsiloxanes.
 3. A rolling bearing comprising a grease composition sealed in the rolling bearing, wherein; the grease composition contains base oil, a thickener, and an organic modified polysiloxane; the base oil is an alkyl diphenyl ether; the thickener is a urea thickener, and the grease composition contains 10 to 30 mass % of the thickener based on a total amount of the base oil and the thickener; and the grease composition contains 0.2 to 10 mass % of the organic modified polysiloxane based on the total amount of the base oil and the thickener.
 4. The rolling bearing according to claim 3, wherein the organic modified polysiloxane is at least one selected from aralkyl modified polymethyl alkyl siloxanes and polyether modified polydimethylsiloxanes. 